This invention relates to the oxidative removal of hydrogen sulfide from gaseous streams with the use of a chelated iron redox catalyst system. Specifically, the invention relates to the stabilization of such a system for operation at low pH in the range of 3.5 to 5.
The removal of hydrogen sulfide and alkyl mercaptans from liquid and gaseous streams, such as the waste gases liberated in the course of various industrial chemical processes, for example, in the pulping of wood, and in petroleum refining, has become increasingly important in combating atmospheric pollution. Such waste gases not only have an offensive odor, but they may also cause damage to vegetation, painted surfaces, and wild life, besides constituting a health hazard to humans. The authorities have increasingly imposed lower and lower tolerances on the content of such gases vented to the atmosphere, and it is now imperative in many localities to remove virtually all of the hydrogen sulfide and alkyl mercaptans, under the penalty of an absolute ban on continuing operation of the plant.
The quantities of hydrogen sulfide and mercaptans in waste gases are often not very high. Dunn U.S. Pat. No. 3,071,433 dated Jan. 1, 1963, indicates that the stack gases obtained in the concentration of black liquor, the waste pulping liquor of the kraft pulping process, contain from 500 to 2,000 parts per million of hydrogen sulfide. However, hydrogen sulfide can be detected by humans at a concentration of approximately 0.01 part per million. The result is that an extremely efficient process for the removal of hydrogen sulfide and alkyl mercaptans is required for effective capture of small amounts of these materials.
The use of chelated iron redox catalysts for the oxidative removal of hydrogen sulfide from gas streams and conversion of the hydrogen sulfide to elemental sulfur is well known in the art. Although the use of the chelated iron catalyst system at a pH ranging from 3 to 11 has been disclosed, the prior art has consistently emphasized the advantages of operating such a catalyst system at substantially neutral or alkaline pH. In such processes, a gas stream containing hydrogen sulfide is contacted with an aqueous solution of chelated ferric ion. The solution absorbs the hydrogen sulfide and converts it essentially quantatively to elemental sulfur which can be separated from the solution by filtration, centrifuging, or the like. The ferric ion, which is reduced to ferrous ion by reaction with the hydrogen sulfide, is then regenerated by contacting the solution with a gas containing elemental oxygen, such as air. The art has stated that both the absorption of hydrogen sulfide and the regeneration of the solution with elemental oxygen occur most efficiently at neutral or alkaline pH.
However, it has been recognized that one disadvantage of operating at neutral or alkaline pH is that a portion of the sulfur, up to several percent, can be further oxidized to various oxides of sulfur. These oxides of sulfur are undesirable by-products and have the effect of lowering the pH of the solution, thus requiring the use of buffers or continued addition of alkali to maintain the solution within the desired alkaline pH range.
The art has recognized an advantage in the use of the chelated iron catalyst system at low pH, namely, that the objectionable oxides of sulfur are not formed. However, a disadvantage of operating at low pH is that many of the best iron chelates such as the iron-nitrilotriacetic acid and the iron-ethylenediamine tetracetic acid chelates are unstable at such low pH and tend to precipitate from solution, thus lowering the effectiveness of the catalyst. For this reason the art has directed its attention to operation in the neutral and alkaline pH regions. A number of approaches have been described for minimizing the formation of oxides of sulfur when operating at alkaline pH.
U.S. Pat. No. 4,036,942 to Sibeud, et al describes a process for removing hydrogen sulfide and alkyl mercaptans from gaseous streams by reaction with oxygen in the presence of a polyvalent metal chelate with an amino acid in an aqueous solution containing an organic amine. Aminopolycarboxylic acid chelating agents of the alkylenediamine and phenylenediamine types are described, such as ethylenediamine tetraacetic acid. The use of aliphatic, alicyclic, and heterocyclic primary, secondary and tertiary amines is also described. The patent states that the oxidation of ferrous ion to ferric ion by oxygen is slow at pHs of from about 1 to 5. Thus the patent teaches operation at a preferred pH of 6.8 to 10. The addition of an amine to the chelated iron solution is said to inhibit the formation of acidic oxides of sulfur at a pH in the preferred range of 6.8 to 10. The use of an amine is also said to enhance the absorption of hydrogen sulfide by the catalyst solution.
U.S. Pat. No. 4,009,251 to Meuly describes a process for removal of hydrogen sulfide from gas streams with an aqueous solution of iron chelated with an aminocarboxylic acid such as nitrilotriacetic acid or ethylenediamine tetracidic acid at a pH in the range of 3 to 11, preferably 7 to 11. The formation of acidic oxides of sulfur with the consequent drop in the pH of the catalyst solution during use is inhibited by the addition of an alkali metal, alkaline earth metal, ammonium, or amine salt of a non-oxidizing acid having a pH within a range of from about 1.2 to about 6. Such acids include formic acid, citric acid, acetic acid, and propionic acid. The use of aliphatic, alicyclic, and heterocyclic primary, secondary, and tertiary amine salts of such acids is also described. Although the patent states that the pH of the system should be within a range of from about 3 to about 11, the most efficient range is said to be be from about 7 to about 11, preferably from about 8 to about 10. The patent also states that a free organic amine can be used to adjust the pH of an acidic chelate solution to within this range.
U.S. Pat. No. 3,933,993 to Salemme describes the use of a chelated iron solution having an iron concentration of at least about 0.5 moles per liter and a pH greater than 7. The use of ethylenediamine tetracetic acid as chelating agent with a buffer such as sodium cabonate or potassium phosphate is described. The patent describes the advantages of using a concentrated solution of the iron chelate but does not suggest the use of such solutions at acidic pH.
U.S. Pat. No. 3,676,356 to Roberts et al describes the use of iron chelated with nitrilotriacetic acid at pH of 5 to 6.5. In order to maintain the desired pH it is necessary to add on a continuous or intermittent basis an alkaline buffering agent, such as an alkali metal or ammonium carbonate or bicarbonate or an amine salt, such as a salt of diethanolamine. The use of an iron nitrilotriacetic acid chelate solution containing ammonia or an aliphatic, alicyclic, or heterocyclic primary or secondary amine at a pH of 3.5 to 5 is not described. Although the use of the buffering agent is said to increase the life of the chelate at a pH in the range of 5 to 6.5, the process has the disadvantage that the amount of buffering agent in the solution substantially increases as the process is continued, and the catalyst solution must eventually be replaced.
U.S. Pat. No. 3,097,925 to Pitts, Jr., et al describes the use of iron chelated with an aminocarboxylic acid such as ethylenediamine tetraacetic acid or N,N-dihydroxyethylglycine at a pH of from 1 to 13, preferably 7 to 10. The patent does not address the problem of the instability of such chelates at high concentration and low pH.
British Pat. No. 999,799 states that hydrogen sulfide is absorbed rapidly by an aqueous solution of iron chelated with hydroxyethylenediamine triacetic acid at a pH above about 2.5. Although the patent recognizes that an organic base such as triethanolamine can prevent precipitation of ferric hydroxide if a chelated iron solution is used at a comparatively high pH value, there is no suggestion that the use of primary and secondary amines can prevent the precipitation of ferrous ion from solution at a comparatively low, acidic, pH.